The present invention relates to switching circuits for power transistors and, more particularly, to an auxiliary resonant power supply for providing power to the transistors and other circuit elements in the switching circuit.
FIG. 1 illustrates a conventional circuit in which a switching circuit 10, comprising, for example, an IR 2110 monolithic bridge driver (available from the International Rectifier Corporation, the assignee herein) drives first and second, series-connected power transistors 12 and 14, for example, power MOSFETS. As is well known, anti-parallel diodes are connected across each of transistors 12 and 14 and are denoted by X. For its own operation, the switching circuit 10 comprising a monolithic bridge driver requires a pair of low-voltage supplies V.sub.DD and V.sub.CC, as shown. Low current logic circuitry in the monolithic bridge driver 10 is powered from V.sub.DD and does not consume much current. The circuitry connected to V.sub.CC draws the current needed to supply the gate charge of the MOSFETs or IGBTs represented by the driven power transistors 12 and 14. This current can be significant. For example, to drive IRF450 HEXFETs (transistors supplied by International Rectifier Corporation) at a frequency of 500 Khz requires a current I.sub.CC : ##EQU1##
There is a need for a simple efficient low cost power supply to furnish this current, at a typical voltage range of 12 to 18 volts.
Prior art methods of implementing the required auxiliary power supply are illustrated in FIGS. 2(a)-2(d). The circuit of FIG. 2(a) uses a "dropping" resistor 18 to drop the relatively high voltage at the main DC bus 20 to a lower voltage that is regulated by the zener 22 and delivered from the storage capacitor 24. This can be practical where the required current is not too high, as the dissipation in the resistor 18 will not be excessive. For example, two of the aforementioned IRF450 HEXFET transistors operating at 3 kHz would consume an average current of about 0.75 mA from V.sub.CC. Allowing for operation over a range of DC bus voltage from 200 to 400 volts, the corresponding maximum dissipation in the dropping resistor 18 would be about 600 mW, which is quite practical. At 500 Khz, however, the corresponding maximum dissipation in the resistor 18 would be about 100 watts, which is unacceptable.
The prior art circuit in FIG. 2(b) uses a "buck" converter design. The "buck" converter design is efficient and provides a well-regulated power supply. By comparison with the to-be described circuit of the present invention, the buck converter may generally not be cost effective, though it does offer better performance, for example, at start-up.
The bridge circuits of FIGS. 2(c) and 2(d) produce their auxiliary power from the AC line. Both circuits are viable only where AC line power is available, but even then, they require quite large and costly components.
In lieu of a buck converter, the present invention relies on the concept of employing a far simpler LC resonant circuit to charge an output low voltage storage capacitor.
Several patents relating to the switching power supply field describe series LC circuits. For example, U.S. Pat. No. 4,184,197 discloses a DC-to-DC converter using two inductances, one in series with the input source and the other in series with the output load. A capacitor is used with a switch (e.g. a transistor) between the inductances. The operation of the circuit, described at column 5, lines 20-37, of the patent, is significantly different from that of the present invention.
U.S. Pat. No. 4,654,769, discloses a DC-to-DC converter with a series LC circuit operating in conjunction with switching transistors Q.sub.1 and Q.sub.2 (see FIG. 2, in particular). However, the configuration/operation of the circuit (described at column 3, line 57 to column 4, line 14) is based upon the previously discussed patent, and differs significantly from the present invention.
U.S. Pat. No. 4,736,284 discloses a switching power circuit with an output delivered from a diode/capacitor configuration. A series LC resonant circuit, connected to supply charging current to an output capacitor is not disclosed.